Micro-electro-mechanical systems (MEMS) are chip-scale structures that can convert mechanical effects into measurable quantities, e.g. electrical or optical signals. In particular, the invention described herein pertains to opto-mechanical systems (optical MEMS) in which nanometer-level displacements are measured optically. There is a variety of current techniques that have limitations and disadvantages when compared to the present invention, which will be described.
One straightforward displacement measurement technique known in the prior art is off-chip displacement detection. In this technique, light can be focused on the tip of a cantilever. The cantilever can be coated with metal so that light can be reflected and measured with a position-sensitive photodetector, and any cantilever motion modifies the detected optical signal.
A more sensitive method for off-chip displacement detection can split the laser light into two paths: one can be the reference and the other can be the displacement signal that includes the optical path of light that reflects off the cantilever. This approach can provide high sensitivity because the reference and signal laser beams interfere and the interference is sensitive to changes in optical path length (Michelson interferometer). However, it has been found that under ideal conditions the methods described have similar displacement resolution.
The two off-chip displacement detection techniques are simple and can be implemented in a lab setting using readily available components. However, the readout mechanism requires accurate optical alignment between off-chip components (e.g. laser, photodetector) and on-chip structures (e.g. mechanical cantilever). This limits the applicability in settings outside of the lab, where accurate alignment may be difficult to achieve. Furthermore, off-chip detection may limit the ability for large-scale integration of many devices.
Another displacement measurement technique known in the prior art is on-chip displacement sensing with optical cavities. A microbridge (alternatively, a cantilever) can act as a movable mirror underneath which is placed a second fixed mirror. The mirrors can form an interferometer whose transmittance (T) and reflectance (R=1−T) are sensitive to the microbridge displacement (Δz):T∝1/[1+(2F/π)2 sin2(φ/2)]  (1),where F is the cavity finesse and the phase is φ=φ0+2π(Δz/λ). From equation (1) the change in transmittance can be a function of displacement and the sensitivity is increased by the F. The finesse depends on the mirror reflectivity (r) as F=πr1/2/(1−r)>>1. In contrast, for a Michelson interferometer F≈1.
Although cavities can increase the displacement sensitivity, they have drawbacks. The fabrication of high-reflectivity mirrors is challenging, especially in an integrated chip-scale structure. High-finesse F implies a narrow optical bandwidth, which limits the operating wavelength. Furthermore, most materials are temperature-sensitive (the refractive index varies with temperature) so that the optical narrowband operation limits the temperature range at which such a sensor can operate. Finally, most micromechanical Fabry-Perot cavities are created using an approach where the cavity is formed parallel to the wafer plane (i.e. surface-normal cavity). Although multiple sensors (microbridges or cantilevers) can be fabricated on-chip, there will be a tradeoff between the number of sensors and the cavity finesse so that compromises have to be made between large-scale integration and sensitivity.
On-chip displacement sensing using cantilever waveguides is another displacement measurement technique known in the prior art. This approach can enable large-scale integration and optical readout. A cantilever optical waveguide can function as a light guide and simultaneously as a mechanical structure. The cantilever waveguide can be initially aligned with a second fixed waveguide. As the cantilever deflects (either in-plane or out-of-plane) the amount of light that is transmitted to the second fixed waveguide is modified as equation (2):
                        ∫                  -          ∞                ∞            ⁢                                    ϕ            1                    ⁡                      (            x            )                          ⁢                              ϕ            2                    ⁡                      (            x            )                          ⁢                                  ⁢                  ⅆ          x                          2where Φ1(x) and Φ2(x) describe the fundamental optical mode field in the cantilever and fixed waveguides, respectively. For identical input/output waveguides the deflection of the cantilever tip modifies the transmittance as T(Δx)=exp[−(Δx/w0)]2, where 2w0 is the optical mode width.
This technique can enable the optical interrogation of many cantilevers and their respective displacements using a single optical input and output, since light can be guided, split, and recombined on-chip using integrated waveguides. However, the shortcoming of this approach is its limited sensitivity. When the cantilever and fixed waveguides are perfectly aligned, the displacement sensitivity is at its minimum. The peak sensitivity occurs for a slight initial misalignment (Δx0). In practice, however, it is difficult to operate at peak sensitivity due to fabrication limitations and intrinsic stresses that result in cantilever misalignment; and even when the initial misalignment is Δx0, this technique is typically less sensitive than interferometry. Fabrication is also challenging, since the gap between the input and output waveguide needs to be minimized to ensure low optical losses and the waveguide mode (w0) should be made small to ensure high displacement sensitivity.
Another type of on-chip displacement sensing using evanescent field sensors can take advantage of the evanescent optical field in a waveguide. The evanescent field is the component of the optical field that resides outside of the waveguide core. The evanescent field decays exponentially away from the core; and, therefore, is highly sensitive to the presence and displacement of structures brought into proximity to this field. In this embodiment, an optical switch can be formed from two suspended waveguides. By varying the gap between the waveguides using electrostatic actuation, the optical coupling via the evanescent field cab be modified. Light can then be switched between one waveguide to the other. A limitation is that the waveguides in a prior demonstration have a diameter that is much larger than the optical wavelength (width=2 μm, or approximately 4λ for nCORE=3.2 and λ=1,550 nm), so that the evanescent field is small and large displacements (Δx≈1,000 nm) are required for switching.
In a second evanescent field device, a circular optical cavity (“microring” resonator) can form an optical filter. A dielectric slab can be brought into close proximity to the surface of the cavity so that it interacts with the evanescent optical field. The result is that the effective refractive index of the microring is increased as the dielectric perturber moves closer to the microring surface. By increasing the effective index, the filter transmission is modified and the resonance wavelengths are shifted. In principle, this structure could be used to measure displacements. However, the waveguides are still relatively large (thickness t=330 nm, or ≈λ/2 for nCORE=2.0), which implies a modest evanescent field. Furthermore, the cavity has a narrow optical resonance line width, which makes the device optically narrowband. Consequently, the device is highly temperature sensitive and may limit the applicability of such a sensor to laboratory environments, where environmental conditions are precisely controlled.
In the prior art, other groups have demonstrated evanescent field devices and have shown displacement sensing, but the waveguides were large (width>λ for the in-plane displacement sensor in and width>λ/2 for the in-plane displacement sensor in). Such optically large structures limit the evanescent field and displacement sensitivity. Furthermore, these devices utilize silicon-on-insulator (SOI) waveguides, whose material absorption results in self-heating and buckling of the waveguide leading to displacement measurement errors. Silicon also has a large thermo-optic effect (dn/dT≈2×10−4/° C.) that will introduce temperature-induced phase shifts, so SOI waveguides are not optimal for high-resolution sensing.
Previously, thin silicon waveguides (e.g., thickness t=110 nm, or ≈λ/4 for nCORE=3.5 and λ=1550 nm) have been demonstrated in the prior art, and used for demonstrating optical forces and measuring displacements of a suspended silicon bridge. However, these waveguides exhibited relatively large loss (5 dB/cm) and large absorption (0.2 dB/cm) that leads to self-heating and thermo-optic phase shifts (dn/dT≈2×10−4/° C.) that complicate any displacement measurement that relies on index perturbation. Furthermore, such thin (e.g., 110 nm) silicon waveguides are fabricated from silicon-on-insulator wafers, which are expensive compared to standard silicon wafers. Finally, the relatively large index of silicon (nSilicon≈3.5 near λ=1550 nm) requires relatively small waveguides (width<500 nm) in order to maintain the single-mode condition. This increases the need for high fabrication accuracy, which increases cost.
Accordingly, there remains a need in the art to develop a displacement sensor system with a waveguide that can enable a large evanescent field; exhibit relatively low loss; be relatively easy and cheap to manufacture; and be more suitable for field settings, in which the environmental conditions are difficult to control compared to in a lab.